Key questions in avian biology today include: How are bird populations regulated? What factors ultimately affect survival and reproduction? How do environmental conditions during the non-breeding periods of wintering and migration ultimately influence breeding success? How do migrants prepare for and complete their journeys? These questions are applicable across a wide variety of bird species and are critical to our understanding of how bird populations are regulated and what factors need to be considered for conservation concerns.
By looking at how environmental conditions (weather, food, etc.) influence energy demand throughout the annual cycle, we can gain a better understanding of how ecological, behavioral, and physiological factors act as “carry-over effects” from one stage to the next (e.g. of “seasonal interactions”). Throughout our work, plasma hormones, metabolites, immune function, and other measures of body condition are examined provide insights into how well individuals are regulating energy. Stable isotope signatures incorporated into tissues such as feathers, claws, and blood are used as biogeographic markers of where and when within the annual cycle individuals may be encountering challenges to their energy demand.
Our research program is multifaceted but the projects are linked by their overriding shared objectives. While each project has a strong “basic” research foundation, they all have an applied, conservation component that considers naturally-occurring environmental factors as well as those that occur through anthropogenic activities such as land use practices, pollution, and development. Loss of suitable resources may occur at one or more stages of the annual cycle and, ultimately, the work focuses on understanding how these stages are linked.
The major goal is to determine if variation in migration strategies (distance, timing, and pathways) explain differences in population trends within and across species of migratory birds by looking at the energetic and time constraints related to different migration strategies. We use stable isotope signatures as geographic markers of an individual migrant’s origin and or destination when sampled en route at spring or fall migration stopover sites to determine migration distance and link this with measures of body condition. We are interested in the underlying physiological mechanisms behind different migration strategies in a variety of species, but have been focusing on the long-distance migrant, the Blackpoll warbler (Dendroica striata) and the shorter-distance, facultative annual migrants, the Yellow-rumped warbler (D. coronata) and the Dark-eyed Junco (Junco hyemalis).
Blackpolls versus Yellow-rumped warblers
The Blackpoll Warbler exhibits the most dramatic autumn migration of any known Neotropical passerine migrant. This small bird (11-15 g lean body mass) can double its body mass with lipid reserves as it prepares for a trans-oceanic migration over the North Atlantic to winter in South America. In autumn, Blackpolls leave their breeding grounds across the boreal forests of North America to congregate along the New England coast in order to fatten in preparation for migration. The initiation of migration follows a regional cold front after which winds out of the northwest prevail and tropical storms are unlikely. Blackpolls may spend 4-5 days in non-stop flight of 3000 to 4000 km in order to reach the north coast of South America and winter throughout the Amazonian River basin. (To learn more about the unique migration of the Blackpoll warbler, check out Jim Baird’s chapter in Ken Able’s edited book, “Gatherings of Angels”, 1999, Cornell Univ. Press!) In comparison, the Yellow-rumped warbler is a short-distance, primarily overland migrant (relatively few cross the Gulf of Mexico) and is the most northerly wintering warbler in North America. Yellow rumps put on little fat during autumn migration and can make facultative movements throughout winter in response to poor weather and decreases in food supply.
Blackpoll versus Yellow-rump migration physiology
When Blackpolls and Yellow-rumps face similar energy demands during the pre-and early migration period (e.g. on the breeding grounds at Churchill, Manitoba), the two species are indistinguishable from each other in patterns of fat deposition, plasma metabolites of lipogenesis, and corticosterone secretion (Holberton et al., ms in prep). However, during the period when they diverge in migration strategies (New England) they also diverge in physiological condition, corticosterone profiles, and triglyceride levels (Holberton et al., ms in prep). In addition to their ability to put on extensive fat reserves, Blackpolls are more efficient in their use of energy reserves during migratory flight (perhaps by as much as 15-fold) compared to other warbler species (e.g., Blackpoll: 0.008 g body mass loss/h versus Yellow-rump: 0.124 g body mass loss/h, Hussell and Lambert, 1980). The different capacity for energy storage and use in these two congeneric and co-occurring species may be the result of entirely different metabolic pathways, different intensities in which the same pathways are regulated, or a combination of both occurring at different steps in the processes. We investigate the nature of these differences in field and in controlled laboratory studies.
A comparative approach, across and within populations of such species like Blackpolls and Yellow-rumps can help reveal the diverse ways in which physiological mechanisms support variation in migration strategies and provide a greater understanding of how selection has acted on migratory behavior and physiology. And, by using stable isotope markers of biogeographic origin of breeding and/or wintering sites, one can tease apart where and when events affecting energy reserves may occur. This collaboration with Dr. Keith Hobson of the Canadian Wildlife Service, Environment Canada, may ultimately help identify factors influencing different population trends and where resource management efforts should be directed.
The Gulf of Maine region is an important migration flyway for a wide variety of birds, including songbirds, raptors, shorebirds, waterfowl, and seabirds. Understanding the energetic constraints of migrants provides greater insight into how they use the Gulf of Maine region. We are working to identify major flyways and the suitable habitats that may need to be protected in order to support the region’s migrant populations, many of whom are experiencing rapid population declines.
Recently, we have begun to document coastal migration patterns and the use of offshore islands by migrants, through establishing of monitoring sites and performing orientation release tests in the Gulf of Maine region.
During migration, birds must stop to rest and replenish energy reserves at stopover sites. The ability to find suitable food while avoiding predators en route enables birds to resume migration more quickly. With sufficient energy reserves, many birds will reduce migration time by taking more direct routes over inhospitable areas, such as deserts or large bodies of water. Although decades of radar work has shown that the Gulf of Maine is a busy migratory route for a variety of bird and bat species, how these animals use Maine’s coastal areas and offshore islands is not well understood. Recent work revealed a previously undocumented but important migration flyway in the Gulf of Maine. During a fall 2009 bird banding study, by USF&W and National Audubon, on several national wildlife refuge islands, more than 5,500 songbirds representing 75 species were captured. This number far exceeded the numbers of songbirds similarly captured at long-running banding stations elsewhere in the northeast. One of our major objectives will be to document migration patterns and the use of offshore islands as stopover sites for songbird migration in the Gulf of Maine. This will enable the US Fish & Wildlife Service to effectively direct resources towards migratory songbirds in the Coastal Islands Refuge System and will provide critical information to state and federal permitting agencies as they look ahead to coastal and offshore energy development initiatives rapidly growing in the region.
Another component considers how a bird’s decision to navigate the complex Maine coastline can be influenced by its ability to gain enough energy to make efficient migration decisions. Former master’s student, Kristen Covino (MS, 2008), showed that migrants use internal information about available energy stores (current fat and body mass, recent change in fat and body mass, and plasma indicators of energetic trajectory – triglycerides and glycerol) to navigate along the Maine coastline during migration. Once captured on stopover, on the Schoodic Peninsula in fall or on Appledore Island in spring, migrants were released wearing small gel capsules filled with a chemiluminescent liquid that allowed Kristen to observe the bird’s departure (if motivated) and preferred direction (vanishing bearing) if it decided to depart. In general, birds with little or no fat either opted not to initiate a flight, or, if they did take off, oriented in a direction that took them away from the coastline, in a risk averse way. Conversely, birds on a positive energy trajectory and with ample fat reserves showed a preferred direction that may have taken them across water but would enable them to move more directly north or south. More work is underway to look at the internal mechanisms influencing these departure decisions in landbird migrants. Such an approach reveals how migratory songbirds use the region to reach their destinations.